The present invention provides a wheel speed sensor for a road vehicle such as a truck or automobile which is easily installed on the vehicle during the course of mass production of the vehicle, is exceptionally compact, and is well suited for the harsh environment in which it is intended to be operated. Importantly, the wheel speed sensor of the present invention does not require critical adjustments during mounting of the sensor components on the vehicle. The present invention also provides a particularly advantageous method for manufacturing the wheel speed sensor of the present invention.
The sensor of this invention is generally of the variable reluctance type in which the reluctance across an air gap between confronting pole pieces of the rotor and stator is varied by relative movement between the rotor and stator. It is desirable to keep the air gap at a minimum dimension so as to provide the greatest possible output signal from sensor. There are, however, practical limitations on the minimum dimension for a wheel speed sensor air gap since the relatively rotatable parts on which the rotor and stator of the sensor are mounted are subject to deflections and the like which could cause damage to the sensor components if the air gap is too small and the opposed surfaces of the rotor and stator are severely engaged. The present invention advantageously utilizes the inherent deflections between the relatively rotatable parts on which the wheel speed sensor is mounted to set the air gap at a minimum practical dimension. This is accomplished by providing the rotor and stator with confronting surfaces which would not be damaged upon engagement therebetween and by mounting one of the rotor and stator, and preferably the stator, so that it may be moved relative to the other upon engagement between the rotor and stator to establish an air gap which ordinarily prevents engagement between the confronting surfaces of the rotor and stator.
In the preferred form of the present invention, the rotor is a stamping having a plurality of closely spaced circularly distributed openings providing closely spaced circularly distributed portions of magnetic material which are sequentially brought into confrontation with a pole piece on the stator as the rotor rotates relative to the stator thereby providing a variable reluctance path between the rotor and the stator pole piece. The stator includes a body which is substantially flush with the outward face of the pole piece and which is of sufficient dimension relative to the dimension of the rotor openings to avoid intermeshing between the rotor and stator so that occasional engagement between the confronting surfaces of the rotor and stator is non-destructive. In its preferred form, the stator includes a pole piece, a bobbin assembly, and a magnet which are axially aligned in a body molding to form an elongated structure of uniform cross section for convenient disposition in a tubular mounting member. The stator and the tubular mounting member are adapted to provide relative axial movement between the stator and the tubular mounting member. The stator and tubular mounting member cooperate with a resilient member which generally resists or impedes axial movement of the stator relative to the tubular mounting member so that the air gap between the rotor and stator will not change under normal forces applied to the stator consequent vehicle vibration and the like without engagement between the rotor and stator but does not prevent such movement when the stator is engaged by the rotor so that engagement between the stator and rotor may effect movement of the stator to provide the minimum practical air gap. In its preferred form, the resilient member is a tubular elastomeric member which is either configured or prestressed to resist axial movement of the stator until a predetermined axial force is applied to the stator by the rotor. The resilient member preferably provides at least two spaced points of resilient support for the stator. Also, preferably, the stator is provided with a serrated surface which is engaged by the resilient member.
According the the method of the present invention the resilient member is prestressed under simulated operational conditions to set the minimum force which must be applied to the stator by the rotor to cause movement of the stator. In this regard, a member which has approximately the same transverse dimension as the stator so as to simulate the stator is moved relative to the resilient member while in engagement therewith during the prestressing operation and a contemporaneous measurement of the force required to cause such movement is taken. When that force reaches the desired level, the prestressing operation is terminated and the degree of prestress is maintained.